Some important biologically active molecules are cyclic peptides. Two important examples of molecules containing cyclic peptide moieties are valinomycin, important in ion transport, and the antibiotic actinomycin. Other examples of cyclic peptides are found in hormones, toxins, alkaloids etc. Cyclic peptides also have great interest in their study as model compounds in the study of protein conformation under well defined conditions. These cyclic peptides occur with different ring sizes and also contain non protein amino acids like N-methyl and D-amino acids. The study of the three dimensional atomic arrangement of these compounds by X-ray crystallographic method, though for so far, has already yielded very valuable information. The aim of the present project is to investigate by X-ray diffraction technique the detailed structure and conformation of a few selected cyclic peptides.